HttpError: Cast Message to string

In theory PHP should use coerce objects with a __toString method
to string when passing them to a method which has a string-typed
argument, but with the native Exception class this doesn't seem
to be working, so cast explicitly.

Bug: T388171
Change-Id: I310b97164877445837babed2a4c47795f1c2c277
(cherry picked from commit a14ef848f758612ac9d4e8c881ecdbbccf3e1158)

includes/exception/HttpError.php

@@ -47,7 +47,7 @@ class HttpError extends MWException {
 	 * @param-taint $header tainted
 	 */
 	public function __construct( $httpCode, $content, $header = null ) {
-		parent::__construct( $content );
+		parent::__construct( (string)$content );
 		$this->httpCode = (int)$httpCode;
 		$this->header = $header;
 		$this->content = $content;